Crystal growth apparatuses or furnaces, such as directional solidification systems (DSS) and heat exchanger method (HEM) furnaces, involve the melting and controlled resolidification of a feedstock material, such as silicon, in a crucible to produce an ingot. Production of a solidified ingot from molten feedstock occurs in several identifiable steps over many hours. For example, to produce a silicon ingot by the DSS method, solid silicon feedstock is provided in a crucible, often contained in a graphite crucible box, and placed into the hot zone of a DSS furnace. The feedstock charge is then heated using various heating elements within the hot zone to form a liquid feedstock melt, and the furnace temperature, which is well above the silicon melting temperature of 1412° C., is maintained for several hours to ensure complete melting. Once fully melted, heat is removed from the melted feedstock, often by applying a temperature gradient in the hot zone, in order to directionally solidify the melt and form a silicon ingot. By controlling how the melt solidifies, an ingot having greater purity than the starting feedstock material can be achieved, which can then be used in a variety of high end applications, such as in the semiconductor and photovoltaic industries.
The heating element used in the hot zone of a DSS furnace can be either resistive or inductive. In induction-type heating, typically a water-cooled heating coil surrounds the silicon feedstock material, and the current flowing through the coil is coupled to a susceptor or the feedstock material to achieve appropriate heating of the feedstock material. In the case of resistance heating, current flows through a resistive element which heats up, and the heating element can be designed with a particular material, resistivity, shape, thickness, and/or current path to meet operating temperature and power requirements. In silicon ingot production by directional solidification, resistance-type heating systems typically are used.
DSS furnaces are particularly useful for crystal growth and directional solidification of silicon ingots used in photovoltaic (PV) applications as well as for to semiconductor applications. For either type of application, it is desirable to produce large silicon ingots to lower average production costs. However, as larger ingots are produced, it becomes increasingly difficult to control heat flow and distribution throughout the furnace hot zone in order to achieve substantially controlled heating and heat extraction during production of the ingot. If heat flow and distribution are not substantially controlled throughout the process, the quality of the ingot may suffer.
In practice, as the cross-sectional area of ingots becomes larger, furnaces are sometimes designed with multiple heating elements in an effort to better control the distribution and flow of heat and the temperature gradient in different zones. For example, commonly owned U.S. patent application Ser. No. 12/933,300 describes, in part, a DSS furnace comprising a heating system having two heating elements—an asymmetric serpentine-patterned top heating element which distributes heat downward toward the surface of a feedstock-filled crucible and an asymmetric serpentine-patterned side heater which distributes heat inwardly toward the sides of the crucible. Both the first heating element and the second heating element effectively heat, melt, and solidify the feedstock charged in the crucible. However, both distribute heat asymmetrically to the crucible, resulting in a non-uniform heat/temperature distribution that can result in variability in the quality of the resulting crystalline ingots.
While dual heating elements can be used to form larger ingots, the use of multiple components adds to the complexity of the solidification system and makes it difficult to control heat flow and distribution precisely, especially in a production environment. It is desirable, particularly in applications for growing large ingots, to provide multiple heating elements that are capable of achieving substantially even heating of the entire feedstock material contained in the crucible and adequately control heat flow and distribution throughout the furnace hot zone. Therefore, it would be desirable to design a heating system which could provide uniform heat distribution to the feedstock-containing crucible to thereby provide for more consistent crystal quality.